With the recent rapid development of information communication technology, a ubiquitous society based on information communication technology is being realized.
In order to ensure the connection of information communication appliances regardless of time and place, it is necessary to install sensors, in which computer chips having a communication function are mounted, to all facilities in society. Thus, the supply of power to these appliances or sensors is becoming a new issue. In addition, as the kinds of portable appliances such as, for example, mobile phones and music players including Bluetooth headsets and iPods are rapidly increasing, users need to consume time and effort to charge batteries. As a method to solve this problem, wireless power transmission technology has recently received attention.
Wireless power transmission or wireless energy transfer technology is technology for transmitting power from a transmitter to a receiver in a wireless manner using the induction of a magnetic field. An electric motor or a transformer using the principle of electromagnetic induction has been in use since the 1800s, after which time methods for transmitting electric energy by radiating electromagnetic waves such as radio waves or lasers have been attempted. In fact, electric toothbrushes or some wireless razors that we frequently use are charged based on the principle of electromagnetic induction.
At present, wireless energy transfer may basically be categorized into electromagnetic induction, electromagnetic resonance, and power transmission using short-wave radio-frequency.
Electromagnetic induction is technology in which, when current is applied to one of two coils that have been moved close to each other, magnetic flux generated by the coil induces electromotive force in the other coil. The commercialization of this technology is rapidly underway, with small appliances, such as mobile phones, as the center. Electromagnetic induction enables the transmission of power of a maximum of hundreds of kilowatts (kWs) and exhibits high efficiency, but disadvantageously requires that the corresponding appliance be brought close to a charger or the ground because the maximum transmission distance thereof is 1 cm or shorter.
Electromagnetic resonance is characterized in that it uses an electric field or a magnetic field, rather than using, for example, electromagnetic waves or current. Electromagnetic resonance has an advantage in that it is safe for other electronic appliances or the human body because it is almost free from problems related to electromagnetic waves. However, electromagnetic resonance may be utilized only within a limited distance and space, and suffers from somewhat low energy transfer efficiency.
Short-wave radio-frequency power transmission, simply referred to as RF power transmission, is based on the fact that energy may be directly transmitted and received in a radio wave form. This technology is RF type wireless power transmission using a rectenna. “Rectenna” is a portmanteau of “antenna” and “rectifier”, and refers to a device that directly converts RF power into DC current. That is, RF power transmission is technology for changing AC radio waves into DC current, and a recent increase in efficiency causes an active study on the commercialization of RF power transmission.
Wireless power transmission technology may be variously utilized across various industries including, for example, mobile phones and the IT, railway, and consumer-electronics industries.
However, in the case of a conventional wireless power receiving device having a metal body, the magnetic field, transmitted by a wireless power transmitting device, may be absorbed by the metal body, which problematically causes deterioration in wireless power transmission efficiency and the emission of heat from the metal body.